1. A person who is so annoying and dumb that they can no longer be classed as an idiot and therefore a new level of idiot was created for them, the Fucknut.
2. A person who as shit for brains (that's if they lucky).
3. A person who was dropped from a tall building at birth.
4. Also see: Alex Glumac

someone who is considered a pest with less than average intellect. someone oblivious to the fact that nobody wants them around because he's a buster. ie., tells nonsensical jokes, butts into conversations, tries to "one up" everyone, blurts out random facts that have nothing to do with the present topic, etc.
"Look, that buster Joel just walked through the door!"
a reply could be:

"Who the hell invited that fucknut to the party?! Come on, let's hide!"